I love skies like this and in my opinion it is one of the best uses for the Sigma 10-20mm super wide angle lens. The distortion in the clouds is caused by the huge angle of view, but I really like the effect, especially when coupled with the deep blue almost black of the sky behind the clouds. This was taken on my 23rd birthday this year up in the hills of Derbyshire and it was a perfect sky in my opinion.